This invention relates generally to signals mounted on pipelines and used to indicate the passage of pipeline pigs. More particularly, the invention relates to pipeline pig signals that make use of magnet arrangements to trigger the release of a spring-loaded indicator flag. This type of pig signal is commonly referred to as a magnetic intrusive pig signal. An example of this type of signal is described in U.S. Pat. No. 7,861,665 to Rankin, the content of which is hereby incorporated by reference.
A magnetic intrusive pig signal typically includes a plug assembly and nipple combination, or a plug assembly and flange combination, that places the nipple in fluid communication with the interior of the pipeline. An o-ring or other equivalent sealing means is used to contain the pipeline pressure. Located within a cavity of the plug assembly is a trigger mechanism having polar opposite magnets. The trigger mechanism is in communication with the release mechanism of an indicator assembly that includes a flag or other device that indicates the passage of a pipeline pig. As a pipeline pig moves along the interior of the pipeline and past the magnetic intrusive pig signal, the pig urges against the trigger. This causes the lower magnet of the trigger mechanism to travel upward toward the upper magnet. As the upper magnet is repelled by the lower magnet, its mounting shaft urges against a release mechanism which activates the flag of the indicator assembly.
Although magnetic intrusive pig signals provide a number of advantages, this type of signal can be negatively affected from time-to-time by ferromagnetic debris such as metal flakes which are attracted to the lower repulsive magnet of the trigger mechanism. The debris can cause the trigger mechanism to jam because of the close fit and tolerances within the trigger cavity. Additionally, heavy waxy sludge sometimes enters the cavity and causes the trigger mechanism to jam. This can be particularly troublesome in the magnet area of the trigger cavity.
None of the prior art magnetic intrusive pig signals include means for preventing pipeline product, debris and sludge from entering the trigger cavity. Further, pig signals that employ non-magnetic trigger mechanisms have no means for preventing contamination of the trigger cavity.